Zac Vawter, fitted with an experimental "bionic" leg, looks out onto the Ledge at the Willis Tower, Thursday, Oct. 25, 2012 in Chicago. Vawter is training for the world's tallest stair-climbing event ... more

Zac Vawter, fitted with an experimental "bionic" leg, looks out onto the Ledge at the Willis Tower, Thursday, Oct. 25, 2012 in Chicago. Vawter is training for the world's tallest stair-climbing event where he'll attempt to climb 103 flights to the top of the Willis Tower using the new prosthesis. Vawter will put his bionic leg to the ultimate test Sunday, Nov. 4, when he attempts to climb 103 flights of stairs to the top of Chicago's Willis Tower, one of the world's tallest skyscrapers. If all goes well, he'll make history with the bionic leg's public debut. (AP Photo/Brian Kersey) less